The scope of the present invention is that of processes and devices allowing the detection of objects or the determination of the propagation characteristics of an acoustic wave in the ground of an area of land.
The invention is in particular aimed at a process and device enabling the detection of mines buried in an area of land.
Mine detecting processes and devices using magnetic means are known. These devices implement a generator and conductive coil. When a conductive material is in the vicinity of the coil, the current flowing through the latter is disturbed and a detection signal is supplied by an electronic processing circuit.
Such detection devices oblige the detection coil to be brought into the vicinity of the mine, thereby endangering the person responsible for handling the detector and thus making demining operations both long and difficult.
In addition, the detector risks being activated by magnetic objects other than the mines being searched for thereby causing false alarms which slow down the demining operations even more.
Lastly, such processes and devices are ill-adapted to the detection of modern mines which hardly use any magnetic materials.
A detection process and device are known elsewhere, notably by patent FR2696573, which are based on the principle of the temporal reversal of an acoustic wave.
This process implements a certain number of acoustic transducers (emitters/receivers) which enables an acoustic beam to be focused, practically automatically, on an object whose exact location is unknown.
The analysis of the signals picked up after a certain number of iterations enables a wave front to be determined wherein the crest or focal point indicates the location of the object.
Such a process is particularly well-adapted to medical imagery and enables the exact location of stones or tumors in the human body to be determined.
Indeed, in this case the transducers are placed at relatively small distances from the objects to be detected (less than 200 mm) and the propagation speed of the sound waves remains roughly the same in every direction of the area to be explored.
This process is, however, ill-adapted to the detection of objects in the ground and notably in land of a large surface area (in the region of 5 to 10,000 m.sup.2).
In fact, the numerous heterogeneities present over such a surface area of land will vary the propagation speed of the sound waves in great proportions, making it impossible to determine a wave front which has a crest whose position is sufficiently well defined.